Hydraulic fluid systems on heavy machinery conventionally comprise vane pumps and/or piston pumps and associated lines, filters, and reservoirs. The pumps provide a high pressure hydraulic fluid to actuators, motors, and/or hydraulic cylinders to provide both motion and positional control of the machinery parts. The high pressure vane pump requires an additive in the hydraulic fluid to provide anti-wear properties and oxidative stability. The preferred additives for vane pumps are the zinc dialkyldithiophosphates. However, these compounds are detrimental to sliding steel-bronze interfaces in the piston pump leading to early, catastrophic failure of the piston pump. Consequently, separate lubricant compositions have been developed for high pressure hydraulic piston pumps and vane pumps. Neither of these separate compositions are satisfactory for both types of pumps. Vane pumps require lubricants containing zinc dithiophosphate extreme pressure additives which however corrode the copper alloy parts of a piston pump. Turbine oils are satisfactory for use in piston pumps but do not provide sufficient chemical reactivity to prevent wear of steel parts in vane pumps. A single hydraulic pump lubricant is desirable, especially for those applications in which both types of pumps draw their lubricant from the same sump.
In the past, various natural and synthetic lubricants compounded with zinc dialkyldithiophosphates have been used as a single hydraulic pump lubricant. These lubricants include mineral oils, paraffinic hydrocarbons, esters of both dibasic and polyol types, and similar materials. However, piston pumps lubricated by these compositions soon fail, due to worn-out bronze load-carrying surfaces.